The present invention relates to high-pressure discharge lamps, and more particularly metal halide high-pressure discharge lamps having structure for containing discharge vessel fragments in the event of discharge vessel rupture.
High-pressure discharge lamps which include a metal halide in the ionizable material are well known. These lamps typically have an ionizable material comprising mercury, a sodium halide and a metal halide contained within a discharge vessel. During lamp operation the ionizable material is vaporized and partially ionized by an electrical discharge through the vaporized material within the discharge vessel to emit light. Ultraviolet radiation is also produced by the discharge. The internal pressure within the discharge vessel is greater than one atmosphere and typically several atmospheres.
Because of the high internal pressure within the discharge vessel, care is taken to design the vessel to withstand such pressures. Notwithstanding good design practice applied to the discharge vessel, on occasion one may fail. If the discharge vessel ruptures the pressure differential to which fragments of the discharge vessel are subjected can accelerate the fragments with sufficient force to penetrate the lamp outer envelope.
One measure taken for preventing discharge vessel fragments from escaping the lamp has been to provide a transparent glass shield around the discharge vessel. U.S. Pat. No. 4,281,274 (Bechard et al) discloses such a lamp. This lamp comprises a typical quartz discharge vessel used in metal halide lamps, and a cylindrical sleeve axially aligned with the discharge vessel and surrounding it. The cylindrical sleeve is a borosilicate hardglass. The patent discloses that the hard glass sleeve performs three functions: temperature control, fragment containment and ultraviolet radiation shielding. The patent describes experimental evidence that the hard glass cylindrical shield must be positively biased in order to minimize the loss of sodium from the discharge vessel.
British patent specification 495,978 accepted Nov. 23, 1938 (based on application No. 16451/37 dated June 14, 1937) also shows the use of a thick auxiliary glass jacket to provide rupture protection in a high pressure metal vapor discharge lamp. No mention is made as to how the auxiliary glass jacket might affect sodium loss from the lamp discharge vessel.
Another containment technique in the event of discharge vessel failure is the use of a wire mesh surrounding the discharge vessel. A lamp utilizing this technique is disclosed in German patent document No. P6118 D published Mar. 5, 1953 (Ulrich W. Doering). This patent document discloses an ultraviolet lamp having a discharge vessel which is not housed within an outer lamp envelope. In order to contain discharge vessel fragments in the event of a rupture, a cylindrical wire mesh body is mounted surrounding the discharge vessel. The lamp is evidently a high-pressure mercury vapor lamp, and there is no concern for the problem of sodium loss and how that might be affected by the wire mesh.
The use of a wire mesh containment shield is also disclosed in U.S. Pat. No. 4,625,140 (Gagnon). This patent discloses a tungsten halogen lamp comprised of an inner light source capsule and an outer lamp envelope. The inner light source capsule is actually a tungsten halogen lamp which operates at high temperature and high internal pressure. The tungsten halogen lamp does not contain sodium, and there is no consideration of how the wire mesh might affect sodium loss if used in a metal halide lamp.
Metal shields show promise in providing effective and reliable protection against discharge vessels ruptures in electric lamps. However, ultraviolet radiation incident on a metal shield will cause the emission of photoelectrons from the metal. These photoelectrons can collect on the lamp discharge vessel to impart a negative charge to it and accelerate the diffusion of positive sodium atoms through the discharge vessel wall. Before a metal shield can be incorporated in discharge lamps that contain sodium, such as metal halide lamps, measures must be taken to avoid exacerbating the sodium loss problem by the incorporation of a metal shield within the lamp outer envelope.
Accordingly, an object of the invention is to provide a practicable metal halide discharge lamp having an internal metal shield for the discharge vessel in a manner that does not accelerate sodium loss so as to diminish lamp performance.